The present invention relates in general to travelling wire EDM apparatus and methods, and more particularly to an apparatus and method for effecting a cut in a workpiece with an electrode wire inclined at an angle relative to the workpiece.
While effecting a cut in a workpiece by way of an electrode wire, it is often necessary to incline the electrode wire relative to the workpiece at an angle other than a right angle. If, in the course of the cut, the inclination of the electrode wire must be changed, or if the direction of feed travel of the electrode wire into the workpiece must be changed, different portions of the electrode wire are caused to travel in the direction of feed into the workpiece along machining paths of different lengths between the beginning and the end of the cutting operation.
It is evident that in the course of effecting a cut in a lateral face of a workpiece to obtain, for example, a truncated conical part, the length of the wire trajectory along the perimeter of the smaller base of the truncated cone is shorter than the length of the trajectory along the perimeter of the larger base. Similarly, while cutting a pyramidal workpiece and reaching the end of a cutting pass on an angled or tapered surface, the feed of the electrode wire into the workpiece must be interrupted when reaching the recessed corner of the workpiece until the cut effected by the electrode wire in the workpiece reaches the tip of the corresponding overhanging corner, or even until it has progressed beyond the overhanging corner, up to a position wherein the inclination of the plane of the cut is returned to a minimum value.
In order to simplify the following description of the apparatus and method and avoid continuously referring to instantaneous distances measured along the portion of the electrode wire momentarily held between the wire support and guide members, the positions occupied by the electrode wire will be described as if the electrode wire was not displaced longitudinally, although it is well known by those skilled in the art that the electrode wire must be continuously renewed at the machining zone in order to compensate for the electro-erosion of the wire during machining and that, consequently, the electrode wire is constantly travelling along its longitudinal axis between its support and guide members.
Because the width of the cut in the workpiece varies as an inverse function of the speed of advance, or lateral displacement of the electrode wire into the workpiece, such lateral displacement being hereinafter referred to as the "feed" of the electrode wire, if different portions of the electrode wire are fed into the workpiece at different speeds, there results an excessive material removal rate where the speed of feed of the wire is low and too small a material rate where the speed of feed of the wire is higher. An extreme case exists when a conical volume is cut from or in the workpiece. The machined surface areas proximate the tip of the conical volume are subjected to excessive material removal causing a rounding of the conical shape because the speed of feed of the electrode wire into the workpiece is practically nil at the tip of the conical volume during machining of the peripheral surface of the cone.
In order to compensate, to a certain degree, for such inaccuracies in dimensions and shape and avoid the effect caused by the localized lack of feed of a portion of the electrode wire when reaching the lateral edge of workpieces cut with a taper or bevel, German patent publication No. DE-2818058 provides for a progressive change of the angle of inclination of the electrode wire within to the plane of the cut, such that all the portions of the whole active length of the electrode wire reach a corner simultaneously, i.e. such that the wire is oriented substantially parallel to the line representing the tip of the corner when the wire reaches the corner, even though the wire may have had a different orientation at the beginning of the cut. In this manner, complete stoppage of the feed of the wire when reaching the recessed portion of the corner is avoided. However, such an arrangement permits only to spread over the length of the cutting path the machining errors otherwise concentrated in the vicinity of a corner. This is due to the fact that the speed of feed of the wire into the workpiece varies for different portions of the wire along the portion of the cutting path during which inclination of the electrode wire progressively varies, and, consequently, the material removal rate varies correspondingly, resulting in machining errors.